Is it Hard or Firm? Decide for yourself....
We had to work this one out for ourselves when one of our customers seriously bent a B767 fuselage in what the pilot, when challenged, described as a "firm touchdown". The pilot had declined to make a tech log entry despite the evidence of a severely distorted fuselage.
In the subsequent argument, conducted while Boeing carried out seven million dollars worth of repairs, we were asked to get the touchdown 'G' from the DFDR. The recorded vertical acceleration was a mere 1.98G so we asked Boeing for a clear definition of a hard landing in terms of 'G'. Boeing replied that, as the peak value is a spike of very short duration compared to the sampling rate of vertical acceleration, there is no way of determining actual vertical acceleration at impact from a DFDR readout. Boeing therefore define a hard landing as a landing which, in the opinion of the crew, is a hard landing. Signs of damage may also help in determining the classification.
In view of the extent of the damage we therefore had no hesitation in re-classifying the reported firm touchdown as a hard landing, despite the relatively mild 'G' figure. The fact remains that there is no known method of extracting the true peak value of vertical acceleration from a DFDR readout and such readouts cannot be relied upon to determine the need for a full hard landing check i.a.w. the Maintenance Manual.
Engineers and aircrew are obliged to use their judgement.